The invention relates to radio frequency (RF) driven plasma ion sources, and more particularly to the RF antenna and the plasma chamber, and most particularly to an ion source with a sputtering converter to produce negative ions.
A plasma ion source is a plasma generator from which beams of ions can be extracted. Multi-cusp ion sources have an arrangement of magnets that form magnetic cusp fields to contain the plasma in the plasma chamber. Plasma can be generated in a plasma ion source by DC discharge or RF induction discharge. An ion plasma is produced from a gas which is introduced into the chamber. The ion source also includes an extraction electrode system at its outlet to electrostatically control the passage of ions from the plasma out of the plasma chamber. Both positive and negative ions can be produced, as well as electrons.
Integrated circuit technology utilizes semiconductor materials which are doped with small amounts of impurities to change conductivity. The most common p-type dopant is boron, and common n-type dopants are phosphorus and arsenic. Negative ions have advantages over positive ions in ion implantation, e.g. preventing charging of the target. Furthermore, most existing ion beam implanter machines use the very toxic BF3 gas to form positive boron ions. Thus a plasma ion source of negative ions would be useful for semiconductor applications. It is also advantageous, particularly for low energy beams to form shallow junctions, to implant molecular ions instead of atomic ions, e.g. B2− or B3− instead of B− to reduce space charge effects during transport of the beam. A higher energy molecular ion beam will have the same energy per atom as a lower energy atomic ion beam.
One method of producing negative ions in a plasma ion source is to include a converter in a source of positive ions for surface production of negative ions. One mechanism for negative ion production is sputtering surface ionization. The converter is made of the material to be ionized. A background plasma is formed of a heavy gas, usually argon or xenon. The converter is biased to about 0.5–1 kV negative potential with respect to the ion source walls and plasma. The positive ions from the plasma are accelerated through the plasma sheath and strike the converter. This results in ejection or “sputtering” of particles from the surface. If the work function of the converter material is low, some of the sputtered atoms are converted into negative ions in the surface and are accelerated through the sheath. RF surface sputtering ion sources have been built, but they use cesium to increase the negative ion yields to acceptable levels, and cesium is a difficult material to use. Thus a non-cesiated RF sputter ion source would be desirable.
Unlike the filament DC discharge where eroded filament material can contaminate the chamber, RF discharges generally have a longer lifetime and cleaner operation. In a RF driven source, an induction coil or antenna is placed inside the ion source chamber and used for the discharge. However, there are still problems with internal RF antennas for plasma ion source applications.
The earliest RF antennas were made of bare conductors, but were subject to arcing and contamination. The bare antenna coils were then covered with sleeving material made of woven glass or quartz fibers or ceramic, but these were poor insulators. Glass or porcelain coated metal tubes were subject to differential thermal expansion between the coating and the conductor, which could lead to chipping and contamination. Glass tubes form good insulators for RF antennas, but in a design having a glass tube containing a wire or internal surface coating of a conductor, coolant flowing through the glass tube is subject to leakage upon breakage of the glass tube, thereby contaminating the entire apparatus in which the antenna is mounted with coolant. A metal tube disposed within a glass or quartz tube is difficult to fabricate and only has a few antenna turns.
U.S. Pat. Nos. 4,725,449; 5,434,353; 5,587,226; 6,124,834; 6,376,978 describe various internal RF antennas for plasma ion sources, and are herein incorporated by reference.